A process for making a thermoplastic foil with a wide-outlet or long-slit die, which has at least one outlet-gap-defining adjustable die lip, an opposing gap-bounding lip and a row of piezotranslators distributed over the length of the outlet-gap-defining adjustable die lip described in the commonly owned U.S. Pat. Nos. 4,594,063 and 4,721,447.
Each piezotranslator is part of a control loop, acts as a positioning member or effector and is associated with an individual section of the outlet-gap-defining adjustable die lip.
At least one actual value of the foil thickness of the thermoplastic foil formed is measured with a measuring device and is fed to a controlling processor.
Each of the piezotranslators is controlled by the controlling processor with a controlling variable. These foils are denoted planar or flat foils.
The expression "controlling processor" means a computer in which a controller is an integral part. The computer is programmed so that the manufacture of different plastic foils can be commanded. It sends commands to the controller.
The piezotranslator is a positioning element or effector which operates according to the piezoelectric principle. Thus by application of an electrical potential the length of a piezotranslator is changed directly from a fraction of a millimeter to about a millimeter. The extension of the length of the piezotranslator is a result of changes in the crystal structure of the individual piezoelectric elements which react themselves and together cause the length extension. In this way, the prerequisites for a very exact fine adjustment of the adjustable die lip are provided.
Reaction times in the vicinity of milliseconds and, depending on the voltage or potential, longitudinal extensions in every required size range are attainable.
The measuring device is part of a control loop which measures the thickness of the web of cooled foil. The values thus measured are fed to the controlling processor which calculates and then produces the controlling voltage for the individual piezotranslators.
The output-gap-defining die lip is generally connected to the rest of the extruder by a flexible deformable component or ligature in the scope of the invention and also is elastically deformable in its boundaries. That is also true in this invention. Also, the opposing gap-bounding lip can be formed as described as an adjustable die lip.
In the known process, the controlling variables are fed directly to the piezotranslators upon comparison of the setpoint thickness value with the actual measured thickness value (German Patent 34 27 915). Thus the same control response is effective over the entire width of the plastic foil.
The foil thickness may be measured over the entire width of the foil, e.g. with a measuring unit movable back and forth over the width of the plastic foil, and the piezotranslators associated with the individual sections of the opposing gap-bounding lip are suitably controlled in this fashion but with identical response.
However, it has been found that comparatively large systematic thickness errors can occur. The systematic errors are mainly due to unavoidable manufacturing and assembly tolerances of the mechanical parts and can also be based on special phenomenon in the flow of the thermoplastic material.
They may be minimized currently but that process is very expensive. If the foil is rolled or wound up into a coil, these thickness errors from foil layer to foil layer add in individual regions of the coil and the finished coil shows an incompletely cylindrical surface With disturbing raised rings, namely so-called "barrel-hop bulges". In making the blown foil these disturbing phenomenon can be prevented since the blowing die or the parts of the surface-forming device for the foil tube move cyclically so that the systematic errors in the coil are distributed over the entire width of the coil and any systematic thickenings themselves can no longer add up to disturbing barrel-hoop bulges. In the foil made from plastic which are made with the wide-outlet or long-slit die that is not possible.